The present invention relates to the production and manufacture of coated paper grades that are finished especially in a supercalender in order to increase the smoothness, gloss and other properties of the paper. The biggest benefits are obtained with a base web that includes at least some mechanical fiber. The improved properties of the printing surface improve the final quality of the printed sheet. The printability of paper and the quality of the printed surface are primary quality factors that are valued by the users of paper. Different kinds of calendering methods like soft-nip, opti-load and Janus Concept Calenders are used both on uncoated and coated paper grades as well as paper board grades.
The finest quality of the paper surface has been achieved by treating a base paper sheet by a supercalender. A supercalender comprises a plurality of soft and hard rolls that are arranged on a vertical stack so that each soft roll is between two hard rolls and vice versa. The stack of rolls can be pressed together in order to produce high linear forces between the contacting surfaces of the rolls. The linear forces are often called nip forces. The hard rolls can be heated. The smoothing of the paper surface is achieved by simultaneously subjecting the fiber structure to high pressure and heat. Under the influence of these forces the fibers forming the paper reach their glass transit temperature and the deformation caused by the nip load is permanent. Sliding of the paper surface on the surface of the rolls also causes deformation of the fibers and increases the smoothing effect. Modern multi-nip calenders comprise often soft rolls made of polymer compounds and effective means for controlling independently the nip loads between each nip of hard and soft roll. These modern types of calenders have several benefits over the earlier supercalenders, the main advantages being better controllabilty and runnability.
One important factor that affects the physical behavior of the paper in the calender nip is the moisture of the base sheet entering the calender. When the moisture content of the paper increases, the effect of the heat and pressure on the fibers is enhanced and the smoothing and glossing effect is increased. The caliper (thickness) of the paper diminishes also during calendering and on the wetter parts of the sheet the reduction of caliper is greater. Therefore, the moisture content of the sheet should be even in cross- and machine directions in order to prevent variations of caliper, gloss, smoothness and other properties of the sheet. For this reason paper is dried usually to high dryness before calendering and it is rewetted for example by steam to a desired moisture level. Steam can be also used for leveling the differences of the moisture content if information on actual moisture content of the sheet is available.
The control of moisture during manufacture differs on manufacturing coated and uncoated paper grades. During coating the base web, which has already been dried to a desired moisture content during manufacture, is wetted and dried at least twice. The coating mix is a admixture of water and additives. When the mix is spread on the base web, the web adsorbs some of the water and the water has to be removed from the web by drying. Coating of both sides of the base web requires two coating and drying cycles and if multiple coating layers are desired, the number of cycles increases accordingly. Several wetting and drying cycles inherently mean that the moisture profile of the web is normally more even than the moisture profile of an uncoated sheet that is dried only once. Water absorbed into a coated web exits the web more slowly through the coating layer, whereby the fibers absorb more water which causes more fiber rise in paper of board grades that contain mechanical fiber.
Modern calenders like soft-nip, opti-load and Janus Concept calenders are usually considered modern supercalenders since these also employ multiple calendering nips. These evolution supercalenders feature more sophisticated possibilities to control the nip loads, roll temperature and sheet moisture than traditional supercalenders. The basic operation principle of using high linear load and heat for smoothing the paper is the same as on supercalenders.
In other types of calenders the nip loads and amount of heat used are smaller than in supercalenders. Therefore, the quality obtained by these methods is not as good as that which is obtainable with supercalenders, but the same physical phenomena occur in all kinds of calenders and the effect on the fibers is the same. It must be noted that sliding of the paper on the nip may, in some calender types, have a more significant role than in a supercalender. Especially so called shoe- or long-nip calenders, wherein the sheet is pressed on a roll with a special pressing shoe over a long distance, offer different operational possibilities than calenders in which the processing time in each nip is shorter. However, the actual basic method of using the combination of heat and pressure for finishing the paper surface is always the same.
It is an object of the present invention to provide an improved method of producing coated and calendered paper and paper board.
It is another object of the invention to provide a novel apparatus for producing calendered paper or paper board.
The invention is most suitable for products wherein the base web of the product contains mechanical fiber, for example groundwood fiber.
According to the present invention the calendered sheet is dried into a moisture content that is 4.0% or less, preferably 3.5% or less, in particular 3.2 to 0.1%, calculated from the dry weight of the paper or paper board.
According to one preferred embodiment of the invention the sheet is dried into a moisture content that is 3.5% or less and remoisturized to a moisture content of 4.0% or less.
According to the other aspects of the present invention, the moisture content of the sheet entering the calender from a paper making machine, coater or an unwinder is at least about 3.0%, preferably 3.0 to 8.0%.
The invention is preferably implemented on supercalendered and coated paper grades. The invention may also be implemented on other calendering methods like soft-nip, opti-load and Janus Concept Calenders and for paper board.
Other objects and features of the invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.